With the emergence of an increasing number of non-vertically drilled wells for the exploration and recovery of hydrocarbon reservoirs, the industry today experiences a demand for logging tools suitable for deployment in such wells.
The conventional wireline technology is well established throughout the industry. The basic elements of down hole or logging tools are described in numerous documents. In the U.S. Pat. No. 4,860,581, for example, there is described a down hole tool of modular construction which can be lowered into the wellbore by a wire line. The various modules of the tool provide means for measuring formation properties such as electrical resistivity, density, porosity, permeability, sonic velocities, density, gamma ray absorption, formation strength and various other characteristic properties. Other modules of the tool provide means for determining the flow characteristics in the well bore. Further modules include electrical and hydraulic power supplies and motors to control and actuate the sensors and probe assemblies. Generally, control signals, measurement data, and electrical power are transferred to and from the logging tool via the wireline. This and other logging tools are well known in the industry.
Though the established wireline technology is highly successful and cost-effective when applied to vertical boreholes, it fails for obvious reasons when applied to horizontal wells.
In a known approach to overcome this problem, the logging tool is mounted to the lowermost part of a drill pipe or coiled tubing string and thus carried to the desired location within the well.
This method however relies on extensive equipment which has to be deployed and erected close to the bore hole in a very time-consuming effort. Therefore the industry is very reluctant in using this method, which established itself mainly due to a lack of alternatives.
In a further attempt to overcome these problems, it has been suggested to combine the logging tool with an apparatus for pulling the wireline cable through inclined or horizontal sections of the wellbore. A short description of these solutions can be found in U.S. Pat. No. 4,676,310, which itself relates to a cableless variant of a logging device.
The cableless device of the U.S. Pat. No. 4,676,310 patent comprises a sensor unit, a battery, and an electronic-controller to store measured data in an internal memory. Its locomotion unit consists of means to create a differential pressure in the fluid across the device using a piston-like movement. However its limited autonomy under down hole conditions is perceived as a major disadvantage of this device. Further restricting is the fact that the propulsion method employed requires a sealing contact with the surrounding wellbore. Such contact is difficult to achieve, particularly in unconsolidated, open holes.
Though not related to the technical field of the present invention, a variety of autonomous vehicles have been designed for use in oil pipe and sewer inspection. For example, in the European patent application EP-A-177112 and in the Proceeding of the 1993 IEEE/RSJ International Conference on Intelligent Robots and Systems, a robot for the inspection and testing of pipeline interiors is described. The robot is capable of traveling inside pipes with a radius from 520 mm to 800 mm.
In the U.S. Pat. No. 4,860,581, another robot comprising a main body mounted on hydraulically driven skids is described for operation inside pipes and bore holes.
In view of the known logging technology as mentioned above, it is an object of the present invention to provide a down-hole tool and method which is particularly suitable for deviated or horizontal wells.